Tunnel field-effect transistors (TFETs) can be used under supply voltages enabled by the low subthreshold swing (SS) which is less than 60 mV/dec. Due to the low subthreshold swing, the on/off threshold voltage is low, accordingly the on/off speed increases and the energy consumption decreases. Therefore, the TFET is also called green FET (GFET). However, the TFET has the disadvantage of low drive current when turned-on. By forming a p-type heavily doped layer on the top of the source end of a FINFET structure, a TFET parallel connected to the double-gated MOSFET having channels at two sidewalls of the fin is formed. The TFET dominates the subthreshold current of the device operated in subthreshold region. Thus, the subthreshold swing is low. When turned on, the double-gated MOSFET dominates the on current of the device to avoid the weakness of the TFET. As shown in FIG. 9, which is a view showing the current-voltage curves of the MOSFET and the TFET, the subthreshold swing of the MOSFET is larger than that of the TFET, the saturation current is higher than that of the TFET. Comparing with the MOSFET, the subthreshold swing of the TFET is small, and the saturation current is low. When the combined structure of TFET and the MOSFET charges to an electrode, the charging current increases more quickly than a conventional MOSFET, due to the introduction of the TFET. As shown in FIG. 9, the on/off speed of the TFET is higher than the on/off speed of the conventional MOSFET on fin structure